The application of synthetic polymer chemistry to the field of sports equipment has revolutionized the performance of athletes in many sports. One sport in which this is particularly true is golf, especially as relates to advances in golf ball performance and ease of manufacture. For instance, the earliest golf balls consisted of a leather cover filled with wet feathers. These “feathery” golf balls were subsequently replaced with a single piece golf ball made from “gutta percha,” a naturally occurring rubber-like material. In the early 1900's, the wound rubber ball was introduced, consisting of a solid rubber core around which rubber thread was tightly wound with a gutta percha cover.
More modern golf balls can be classified as one-piece, two-piece, three-piece or multi-layered golf balls. One-piece balls are molded from a homogeneous mass of material with a dimple pattern molded thereon. One-piece balls are inexpensive and very durable, but do not provide great distance because of relatively high spin and low velocity. Two-piece balls are made by molding a cover around a solid rubber core. These are the most popular types of balls in use today. In attempts to further modify the ball performance especially in terms of the distance such balls travel and the feel transmitted to the golfer through the club on striking the ball, the basic two piece ball construction has been further modified by the introduction of additional layers between the core and outer cover layer. If one additional layer is introduced between the core and outer cover layer a so called “three-piece ball” results and similarly if two additional layers are introduced between the core and outer cover layer, a so called “four-piece ball” results, and so on. In addition, so called “dual core” balls have been made in which the central rubber core is made from two discrete sections of the synthetic rubber material each section having different properties such as compression, resilience, hardness and durability. Even more recently, balls have been introduced having both dual cores and additional thin intermediate layers between the dual core and outer cover layer.
The synthetic rubber formulations used for the core compositions of modern golf balls are based on polybutadiene, especially cis-1,4-polybutadiene. In order to tailor the properties of the core, the polybutadiene is often further formulated with crosslinking agents, such as sulfur or peroxides, or by irradiation, as well as co-crosslinking agents such as zinc diacrylate. In addition, the weight and hardness of the core may be further adjusted by the incorporation of various filler materials in the rubber formulation. Thus, there is a great deal of literature concerning such formulation chemistry and the variation of the rubber composition and degree of cross linking such that cores may be produced with the required compression, resilience, hardness and durability.
After core formation, any intermediate layers and finally the golf ball outer cover and are typically formed over the core using one of three methods: casting, injection molding, or compression molding. Injection molding generally involves using a mold having one or more sets of two hemispherical mold sections that mate to form a spherical cavity during the molding process. The pairs of mold sections are configured to define a spherical cavity in their interior when mated. When used to mold an outer cover layer for a golf ball, the mold sections can be configured so that the inner surfaces that mate to form the spherical cavity include protrusions configured to form dimples on the outer surface of the molded cover layer. When used to mold a layer onto an existing structure, such as a ball core, the mold includes a number of support pins disposed throughout the mold sections. The support pins are configured to be retractable, moving into and out of the cavity perpendicular to the spherical cavity surface. The support pins maintain the position of the core while the molten material flows through the gates into the cavity between the core and the mold sections. The mold itself may be a cold mold or a heated mold.
In contrast, compression molding of a ball cover or intermediate layer typically requires the initial step of making half shells by injection molding the layer material into an injection mold. The half shells then are positioned in a compression mold around a ball core, whereupon heat and pressure are used to mold the half shells into a complete layer over the core, with or without a chemical reaction such as crosslinking. Compression molding also can be used as a curing step after injection molding. In such a process, an outer layer of thermally curable material is injection molded around a core in a cold mold. After the material solidifies, the ball is removed and placed into a mold, in which heat and pressure are applied to the ball to induce curing in the outer layer.
Of the various cover molding processes, injection molding is most preferred, due to the efficiencies gained by its use including a more rapid cycle time, cheaper operating costs and an improved ability to produce thinner layers around the core and closely control any thickness variation. This latter advantage is becoming more important with the developments of multi-layered balls with two or more intermediate layers between the core and cover thus requiring thinner layer formation. Such multilayered golf balls are often fabricated with chemically distinct layers to produce various combinations of hardness, modulus and other properties in order to tailor the resulting principal performance categories, including ball velocity, compression, spin and distance. However, in addition to being time consuming and expensive and imparting additional complexity to golf ball preparation, due to their differing chemical compositions, the individual layers often suffer from delamination often manifested as poor shear performance.
One such chemical composition used especially on early golf ball outer cover layers was naturally occurring balata rubber. One deficiency of balata is the ease with which it is cut or sheared leading to low durability of the ball. As an alternative, there were a number of early attempts to incorporate synthetic cis 1,4-polybutadiene rubber, as a layer around a central core or as an outer cover layer. For example, U.S. Pat. No. 3,784,209 exemplifies a ball prepared by forming a center of a first cis 1,4-polybutadiene formulation and forming a outer cover layer around it of a second cis 1,4-polybutadiene formulation and finally curing the resulting golf ball precursor in a compression molding step. Also, U.S. Pat. No. 4,625,964 exemplifies a golf ball having a central core made from cis 1,4-polybutadiene around which was compression molded two half shells previously prepared from another cis 1,4-polybutadiene formulation to form a second layer. The ball was completed by injection molding an ionomer outer cover layer. U.S. Pat. No. 4,714,253 exemplifies a golf ball having a central core formed from a first cis 1,4-polybutadiene formulation followed by forming an second layer from another cis 1,4-polybutadiene formulation. Two pre-formed ionomer half shells were then pressure molded around the two piece solid core to form the final golf ball. U.S. Pat. No. 4,848,770 exemplifies a three piece solid golf ball having a center produced from a highly filled cis 1,4-polybutadiene formulation and second layer produced from a second unfilled cis 1,4-polybutadiene formulation, followed by compression molding two ionomer half shells to form the outer cover layer.
However because of the relatively high viscosity of cis 1,4-polybutadiene formulations at normal injection molding temperatures, which becomes even more pronounced if such formulations also include filler, these formulations are not easily adaptable to traditional thin layer-forming injection molding techniques. Thus the current evolution in golf balls technology favors the use of thermoplastic materials such as ionomers or thermoplastic polyurethane in golf ball covers and intermediate layers, which materials are much more amenable to modern thin layer injection molding techniques.
In addition to the polybutadiene-based synthetic rubbers, another synthetic rubber available for use in golf balls, are the so-called “polyalkenamers”. These synthetic rubbers are unique in that in addition to a liner polymeric component, they also contain a significant fraction of cyclic oligomer molecules, which in turn lowers their viscosity. Compounds of this class can be produced in accordance with the teachings of U.S. Pat. Nos. 3,804,803, 3,974,092 and 4,950,826, the entire contents of all of which are herein incorporated by reference.
To date, this material has been utilized primarily in blends with other polymers. For instance, U.S. Pat. No. 4,183,876 describes compositions comprising 15-95 parts by weight crystalline polyolefin resin and correspondingly 85-5 parts by weight cross-linked polyalkenamer rubber per 100 total parts by weight of resin and rubber. The resulting moldable thermoplastic compositions were said to exhibit improved strength and greater toughness and impact resistance than similar compositions containing substantially uncross-linked rubber. U.S. Pat. No. 4,840,993 describes a polyamide molding compound consisting of a mixture of 60 to 98% by weight of a polyamide and 2 to 40% by weight of a polyalkenamer, wherein the mixture is treated at elevated temperatures with 0.05 to 5% by weight of an organic radical source. No mention was made of the use of such compositions in balls including golf balls.
However, there a number of applications of polyalkenamer blends in game balls of various kinds. For example, U.S. Pat. No. 5,460,367 describes a pressureless tennis ball comprising a blend of trans-polyoctenamer rubber and natural rubber or other synthetic rubbers, e.g. cis-1,4-polybutadiene, trans-polybutadiene, polyisoprene, styrene-butadiene rubber, ethylene-propylene rubber or an ethylene-propylene-diene rubber (EPDM).
Also, U.S. Pat. No. 4,792,141 describes a golf ball comprising a core and a cover wherein the cover is formed from a composition comprising about 97 to about 60 parts balata and about 3 to about 40 parts by weight polyoctenylene rubber based on 100 parts by weight polymer in the composition. This patent also discloses that using more than about 40 parts by weight of polyoctenylene based on 100 parts by weight polymer in the composition has been found to produce deleterious effects.
However, it would be highly advantageous to have an injection moldable rubber composition with the soft feel of a rubber such as balata, but of sufficiently low viscosity to allow the material to be injection molded. It would also be highly advantageous if the properties of such a rubber composition could be tailored by similar formulation chemistry to that which has evolved through the use of crosslinked filled polybutadiene compositions used in core construction
It would also be highly advantageous to be able to form a golf ball based on a single injection moldable rubber composition having different cure packages in order to achieve the desired hardness and or density gradient necessary for optimum ball performance, while also providing high COR and, if used as an outer cover layer, excellent shear cut and delamination resistance.
The present disclosure provides a golf ball comprising an injection moldable polyalkenamer rubber composition wherein the golf ball has a region in the golf ball made from a single polyalkenamer base resins but having one or more hardness regions within the single component region.
The present disclosure also provides processes for preparing a golf ball by sequentially injection molding layers of polyalkenamer rubber compositions with the required curative packages all having a sufficiently low viscosity at and below normal peroxide decomposition temperatures to allow the material to be injection molded to form a golf ball followed by compression molding the resulting golf ball or golf ball precursor to form a region in the golf ball made from a polyalkenamer component but having one or more hardness regions within the single component region.
The present disclosure also provides processes for preparing a golf ball by injection molding half shells of polyalkenamer rubber compositions having a sufficiently low viscosity at and below normal peroxide decomposition temperatures to allow the material to be injection molded to form a half shells all formed from a polyalkenamer as the base resin each half shell comprising different curative packages, followed by combining the half shells around a golf ball core and compression molding the resulting golf ball or golf ball precursor to form a region in the golf ball made from a single polyalkenamer base resins but having one or more hardness regions within the single component region.